Shigella is the causative agent of bacillary dysentery in humans and primates. Over 200 million cases are reported annually and about 650,000 persons die of shigellosis each year. No effective vaccine exists. Recent bioterrorism concerns increase the importance of understanding Shigella pathogenesis since Shigella has been classified as a category B agent on the NIAID's list of priority pathogens for biodefense research. Shigella is a classic example of an invasive, facultative intracellular pathogen that is exquisitely adapted to living within the cytoplasm of the host cell. Moreover, Shigella has evolved mechanisms to replicate within and exit the host while avoiding the host inflammatory response. Our long term objectives have focused on post-invasion aspects of Shigella pathogenesis including molecular characterization of secreted effector proteins and their role in modulating the host immune response and facilitating intracellular bacterial growth and survival. The specific aims of this proposal are to: 1) Characterize the bacterial and host factors responsible for the anti-apoptotic effects of Shigella flexneri infection in epithelial cells; and 2) Characterize the role of the Osp proteins with respect to Shigella pathogenesis and the host immune response. Several models and innovative experimental strategies for testing them are proposed. Genetic, biochemical and molecular and cell biology approaches will be utilized in all aims. This research will fill in important gaps in our knowledge of Shigella pathogenesis. Information on how secreted effectors of Shigella modulate host cell survival (anti- apoptosis) and the host inflammatory response can reveal new targets for the design of therapeutic agents. The results of the proposed studies can serve as a paradigm for understanding similar phenotypic traits of other intracellular bacterial pathogens.